1. Field of the Invention
The present invention relates to a method of and an apparatus for generating a proof of an image generated by a first device, with a second device.
2. Description of the Related Art
For producing a printed material containing a color image, it is necessary to take into account various printing conditions including the type of inks and print sheets used to print the color image, the illumination used to observe the printed material, and various other factors in order to obtain the desired printed material. These printing conditions may be established by actually printing the color image in test under various printing conditions. However, it would not be practical to perform test printing processes under all combinations of various many printing conditions because such test printing processes would require a large expenditure of time and cost.
There has been developed a print proof generating apparatus capable of easily simulating finished states of a printed material without carrying out an actual printing process. The developed print proof generating apparatus operates by converting printing image data supplied to generate a printed material in view of various printing conditions thereby to generate device-independent colorimetric data of expected colors of the printed material, thereafter converting the device-independent colorimetric data into proof image data that take into account the characteristics of the print proof generating apparatus, and then generating a proof based on the proof image data.
Since the print proof generating apparatus allows printing conditions to be freely established by a conversion table which converts printing image data into calorimetric data, it is possible to generate a proof easily and inexpensively and then determine printing conditions capable of obtaining a desired printed material from the proof.
Theoretically, all colors of color printed materials can be formed from three colors, i.e., C (cyan), M (Magenta), and Y (Yellow). However, the inks that provide these colors may not necessarily have ideal spectral characteristics. In order to compensate for the lack of such ideal spectral characteristics, it has been customary to add the ink of K (black) to those of C, M, Y to produce color printed materials with four colors. For producing highly accurate color proofs, the print proof generating apparatus also use four colors of C, M, Y, K.
The device-independent colorimetric data representing colors are expressed by three variables such as colorimetric values L*, a*, b* of the CIE calorimetric system or tristimulus values X, Y, Z. However, the proof image data are expressed by four variables of C, M, Y, K. Therefore, a conversion table for converting colorimetric data into proof image data cannot uniquely determine proof image data because it needs to convert three variables into four variables.
One solution is to determine K of the proof image data based on the colorimetric value L* and then determine remaining C, M, Y of the proof image data from the colorimetric values L*, a*, b*. However, this approach is disadvantageous in that even if the printing image data represents K only as with a printed material containing only characters, proof image data of C, M, Y may possibly be generated from the colorimetric value L*, and even if the printing image data does not contain data representing K, proof image data of K may possibly be generated. When these drawbacks occur, the printed material that is actually produced may not be in agreement with the proof highly accurately.
According to another proposal, a colorimetrically measurable gradation pattern in black only is produced as both a printed material and a proof, the association between the lightness levels of color values obtained by colorimetrically measuring the printed material and the proof is determined with respect to K, and then a conversion table for C, M, Y, K is produced based on the determined association (see Japanese laid-open patent publication No. 11-27553).
The above proposed process does not consider the difference between area percentages and the difference between densities due to the dot gains of the printed material which represents a halftone dot image and the proof. Therefore, if an area gradation image is reproduced as a proof, then the reproduced image may have different gradation characteristics.